We are studying the function of the cytoskeletal protein nonmuscle myosin II in vivo and in cultured cells. Nonmuscle myosin II is composed of a pair of heavy chains and two pairs of light chains and two different isoforms of the heavy chain, NMHC-A and B, are encoded by separate genes. For in vivo studies we generated 3 lines of mice, B-/B-, BdeltaI/B- and BdeltaI/BdeltaI which show complete ablation, a major reduction (greater than 90%) and a moderate reduction (65-90%) of NMHC-B in heart and brain. The reduction in NMHC-B resulted from replacing a neural-specific alternative exon with PGK-Neo (J. Clin. Invest. 105, 663, 2000). These mice show a gene dosage effect with respect to their phenotype, with B-/B- mice developing cardiac myocyte hypertrophy by E12.5 (Proc. Natl. Acad. Sci. USA. 94, 12407, 1997), BdeltaI/B- mice by 1 month after birth and BdeltaI/BdeltaI mice between 7-11 months. Only B-/B- and BdeltaI/B- mice develop a membranous ventricular septal defect. We relate the onset of myocyte hypertrophy in BdeltaI/BdeltaI and BdeltaI/B- mice to decreased NMHC-B in the Z-lines and intercalated discs of the cardiac myocytes, which do not contain NMHC-A (Takeda et al., Cell Motil. Cytoskel. 46, 59, 2000). The neuroepithelial cells in the developing brains of all 3 lines show abnormalities in cell adhesion and cell migration, resulting in the onset of hydrocephalus during embryonic development (B-/B-) or after birth (BdeltaI/B-). Mice with less NMHC-B show more profound brain defects at an earlier age than those that express more NMHC-B.Recent studies have focused on the onset of cardiac myocyte hypertrophy in B-/B- mice. These cells differ in that they do not contain NMHC-A, therefore, ablation of NMHC-B might lead to a defect in cytokinesis. Evidence that this might be the case was found in E12.5 B-/B- mice, which not only demonstrated myocyte hypertrophy in all of the heart chambers, but also manifested a marked increase in binucleated nuclei compared to B+/B+ and B+/B- cardiac myocytes. Our present studies are directed at further analyzing cytokinesis in the B-/B- hypertrophied cardiac myocytes.